The invention relates to a circuit breaker having a bimetal element, and a current bus extending parallel to and within a deflection plane of the bimetal element. The current bus is rigid relative to the bimetal element for supporting a deflection of the bimetal element caused by an action of electrodynamic forces.
Such circuit breakers are disclosed, for example, in EP 0,391,086.A1. There, a U-shaped bimetal element is connected electrically in series with a likewise U-shaped extension which acts as a current bus. The current bus here flanks the bimetal element in such a way that the current directions of the sections of current bus and bimetal element that face one another in the deflection plane of the bimetal element are opposite. Due to these oppositely directed currents, bimetal element and current bus will greatly repel one another, particularly at high currents. Since the current bus is fixed in the circuit breaker housing, the repelling forces act fully on the bimetal element as additional electrodynamic forces in order to bend it outwardly in its deflection plane. The relatively slow, thermally caused deflection movement of the bimetal element is consequently supported by the effect of the electrodynamic forces. Since this effect occurs particularly at very high currents, the turn-off time in the case of a short circuit is thus particularly short. With small currents, the electrodynamic force effect is of only subordinate significance or is not effective at all.
In the prior art bimetal controlled circuit breaker, the bimetal element may be overloaded by currents that are too high. It is destroyed or at least adversely affected in its accuracy and sensitivity of response. Thus reliable operation of the circuit breaker is no longer ensured. To overcome this danger, prior art circuit breakers can be employed only within a very limited spectrum of different current intensities. Under certain circumstances, several circuit breakers must be employed for different current intensities.
In order not to be overloaded at high current intensities, the bimetal element may be made more robust, for example by enlarging its cross section. However, a more robust construction adversely influences its sensitivity and accuracy of response.